The role of predation as a limiting factor of
bellbird (Anthornis melanurai) nest success in New Zealand

Abstract/OtherAbstract
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Nest success, feeding behavior and anti-predator strategies, were studied to
determine if bellbirds (Anthornis melanura) are limited by predation and if pest
control can relieve the pressures of predation so that food supply may become a
limiting factor for bellbirds.
The nest success of bellbirds at the Rotoiti Nature Recovery Project (RNRP),
Nelson Lakes, New Zealand (2002-2004 breeding seasons) was compared with data
from other bellbird nest success studies in New Zealand. Two years of nest
monitoring resulted in a low overall nest success at the RNRP (39 %), an area low in
predator density, compared with 16.4 - 67.2 % found for other sites. One-way anova
comparisons between the nest success of bellbirds in areas grouped by predator
density (absent, low and high) were not significantly different. Regression analyses of
nest success versus predator density (stoat or rat tracking indicies) were also not
significant.
A test of change in bellbird numbers (numbers in February/numbers in
November in the same breeding season) versus rat density (tracking index in
November) showed no significant trend at the RNRP. A test of density dependence
(change in bellbird numbers versus numbers in November) was also not significant.
The percent of time bellbirds spent feeding was significantly higher at the
RNRP compared to Mt. Misery (Nelson Lakes, New Zealand) during the breeding
season only (RNRP = 34 % and Mt. Misery = 23 %). The percent of time bellbirds
spent foraging for food was also significantly higher at the RNRP compared to Mt.
Misery during the breeding seasons (RNRP = 47 % and Mt. Misery = 36 %). This
suggests that predator control at the RNRP has allowed bellbird numbers to increase
so that nesting success may be partly limited by food supply during the breeding
season. However, despite higher densities of bellbird and other bird species at the
RNRP, there was no difference in the percent of time spent feeding (RNRP = 36.7 %
and Mt. Misery = 36.3 %) or foraging (RNRP = 49 % and Mt. Misery = 51 %)
between the RNRP and Mt. Misery during winter months
The nest height of bellbirds was not significantly related to the nest outcome
(excluding nests that failed due to non-predation causes) at either the RNRP or
Kowhai Bush. Nest predation occurred at a range of heights, which suggests
introduced predators are not specialists at any nest height. Bellbirds at the RNRP
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visited their nests more frequently and stayed for shorter periods at the nest compared
with bellbirds at the Cheeseman Valley, consistent with the hypothesis bellbirds alter
their behavior to avoid disclosing their nest position.
The results of this study, although not significant, do indicate that bellbird nest
success is limited by high densities of predators. One-way anova analysis on nest
success during the feeding stage was nearly significant (P = 0.096). Regression
analysis trend lines show an increased negative effect on nest success at higher
predator densities. Few replicates and large variation in nesting success within
treatments affect the power of the results. Bellbirds may persist in areas with high
predator densities due to anti-predator strategies when visiting their nests. With pest
control, ecosystem interactions may be revitalized so that limiting factors other than
predation become important for bellbird nest success.